1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and particularly, to an exhaust gas purifying apparatus provided with a NOx purifying device having NOx adsorbing capacity.
2. Description of the Related Art
The exhaust gas purifying apparatus provided with the NOx purifying device, containing a NOx absorbent for absorbing NOx, is shown in Japanese Patent Laid-open No. Hei 6-10725. In this apparatus, when the amount of NOx absorbed by the NOx purifying device reaches a predetermined amount, the air-fuel ratio of air-fuel mixture supplied to the engine is set to a value on a rich side with respect to the stoichiometric ratio, and the absorbed NOx is reduced.
According to this exhaust gas purifying apparatus, the air-fuel ratio enrichment for reducing absorbed NOx is performed so that a degree of the enrichment may become larger, and the enrichment execution period may become shorter as the exhaust gas temperature becomes higher. This is intended to obtain an appropriate balance between the NOx discharging amount from the NOx absorbent and the amount of reducing components in the exhaust gases, considering that the NOx discharging characteristic of the NOx absorbent changes depending on its temperature, i.e., the NOx discharging speed (discharging amount per unit time period) is comparatively low when the temperature is low and becomes higher as the temperature rises.
In the above-described conventional apparatus, the amount of enrichment is controlled to be small so that an amount of ammonia generated in the apparatus may not increase when the exhaust gas temperature is low. However, if using a NOx purifying device having a capacity for retaining the generated ammonia, it is not necessary to suppress the generation of ammonia. It is rather desirable to increase the amount of ammonia generated, since the retained ammonia can reduce NOx upon the lean burn operation of the engine.